ANTAKYA, Turkey — Syrian rebels seized control of several critical border crossings Thursday as thousands of people fled the rapidly escalating violence in the capital, Damascus, offering fresh evidence that President Bashar Assad’s government is starting to unravel.

Street fighting stretched into a fifth day in Damascus, with government soldiers deploying snipers on rooftops and helicopter gunships in flash-point neighborhoods. More than 20,000 people were reported to have fled into neighboring Lebanon, and activist groups said more than 55 were killed in Damascus and its suburbs a day after a bombing in the heart of the city killed three of Assad’s most senior advisers.

With the veto by Russia and China of a United Nations Security Council resolution that would have imposed sanctions on Syria effectively heralding an end to diplomatic efforts to resolve the crisis, the growing violence seemed to leave little doubt that both sides are gearing up for a fight to the finish.

“There’s no way the armed opposition would go for a negotiated settlement now,” said Jeffrey White, defense fellow at the Washington Institute for Near East Policy.

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Wednesday’s bombing in a key security building in Damascus killed Assad’s defense minister, Dawoud Rajha; the head of his crisis-management cell, Hassan Turkmani; and his brother-in-law Assef Shawkat. The attack left the government looking shakier than at any point in the 16-month uprising.

Rumors that Assad also had been killed were quelled when the president made his first appearance since the attack in a brief segment broadcast on state TV on Thursday showing the swearing-in of the new defense minister, Fahd Jassim al-Freij.

The pictures depicted the two men in a room with an ornate chandelier and tables but did not mention a location, fueling speculation that Assad may have left Damascus.

It was unclear whether the seizure of border crossings into Turkey and Iraq would provide a significant boost to the rebels’ efforts to oust Assad. When rebels briefly seized a Syrian post on the Turkish border a few weeks back, Turkey closed the crossing.

But the seizures offered a symbolic reminder of the government’s vulnerability in the outlying provinces, which have long been in open revolt and where armed rebels hold sway over large portions f territory.

“These operations are important because they boost the morale of our people and are political messages,” Col. Malik Kurdi, deputy commander of the Free Syrian Army, said in a telephone interview from the military refugee camp in southern Turkey where the nominal rebel leadership is based.

Free Syrian Army fighters overran the main Abu Kamal border crossing between Syria and Iraq about 8 p.m. Thursday, tearing down pictures of Assad, burning the Syrian flag and erecting the revolutionary one, according to Farhan Fteikhan, the mayor of Qaim district in the adjoining Anbar province in Iraq.

Seven border posts were also burned, he said, and there was no attempt to resist by Syrian forces, suggesting they had either abandoned their posts or defected to the opposition.

The seizures represented a problem for Iraq’s Shiite-led government, which has offered hesitant support to Assad for fear that the ascendancy of the Sunni-dominated insurgency in Syria would encourage its own Sunni opponents.

Iraq imposed a curfew on the Qaim area to prevent infiltration by insurgents on either side of the border and sent 1,500 soldiers to the border area, Fteikhan said.

Similar scenes unfolded Thursday at the Bab Hawa crossing point on the Turkish border, with rebels seen tearing down Assad posters and firing AK-47s into the air in a video posted online. Opposition activists said rebels took control of the crossing, along the main road to Aleppo, Syria’s second-largest city.

But a rebel on the border who asked to be identified by his alias, Mutassim Sarmadawi, later said the rebels withdrew Thursday night after threats by government forces to launch an attack on the nearby village of Sarmadi.

The incident illustrated the seesawing nature of the conflict in Syria’s countryside, where towns routinely change hands every few weeks and neither side has been able to gain a decisive advantage.

“There are obvious signs that the regime is cracking, but I don’t think things will just tip over,” said Andrew Tabler of the Washington Institute for Near East Policy. The government has not brought all its resources into play and could make greater use of combat helicopters, start using fixed-wing bombers and resort to the use of chemical weapons.

“There are several ways it can accelerate,” Tabler said.

The capital has begun to resemble a battleground, with fighting between rebels and pro-government forces reported in many Damascus neighborhoods Thursday.

Reuters reported that about 20,000 Syrians had crossed the border into Lebanon in the past 24 hours. Wael Khalidi, director of a nongovernmental organization called Syrian Refugees Affairs, said he thought the number was much lower. “We are afraid the Lebanese authorities will stop allowing refugees in and we will be in big trouble,” he said.